Liquid crystals and some of their applications in chemistry - American

REPORT FOR ANALYTICAL CHEMIBTS

LIQUID CRYSTALS AND SOME OF THEIR APPLICATIONS IN CHEMISTRY GLENN H. BROWN LIQUID CRYSTAL INSTITUTE KENT STATE UNIVERSITY KENT, OHIO 44240

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ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969

have the mobility L of liquids and the optical properties of solids. The liquid crystalIQUID CRYSTALS

line state has more order in the arrangement of its molecules than the liquid state, but less than the solid state. Liquid crystals are divided into two major groups. One of these is identified as thermotropic, indicating that this class is prepared by heating. The known thermotropic liquid crystals are organic compounds; a few coordination compounds containing mercury (11) have been synthesized-e.g., bis(p-methylbenzal) -p,p’-diaminomercurybiphenyl. Lyotropic liquid crystals constitute the second major group and are prepared by mixing two or more components. In twocomponent systems involving water, the second component is generally an amphiphile. Lyotropic systems can be large in number and varied in composition. We shall limit our presentation to thermotropic liquid crystals which have had the most interesting developments in chemistry. Thermotropic liquid crystals may be divided into two classes. One of these is identified as the nematic liquid which has spontaneous orientation of the molecules with their long axes parallel (Figure 1A). There is no long-range order in the spatial arrangement of the molecules in nematic liquids. The cholesteric liquid is recognized as a special case of the nematic liquid (Figures 1B and 3 ) . The molecules have an orientation order similar to nematic liquids but superimposed on their parallel orientation is a spontaneous and continuous twist. The second class of liquid crystals is identified as the smectic liquid. The smectic liquid of Type A has the arrangement of the centers of gravity of the molecules in planes perpendicular to the preferred direction of the long axes of the molecules (Figure IC) and therefore perpendicular to the optical axis. There is no spatial long-range order of the molecules in the planes.

Figure 1. A. Schematic diagram of molecular arrangement in uniformly oriented nematic liquid Uniformly orlented structure has infinite-fold symmetry axls

B. Schematic diagram of molecular arrangement in uniformly oriented cholesteric liquid Infinite-fold screw axis

C. Schematic diagram of smectic liquid Infinite-fold symmetry axis

D. Schematic diagram of molecular arrangement in tilted smectic structure E. Schematic diagram of molecular arrangement in twisted smectic structure

Molecular Geometry of Molecules Which Form Thermotropic Liquid Crystals

The kinds of molecules which form liquid crystals generally possess certain features of common geometry even though the compounds may be of a variety of types such as anils, azo compounds, azoxy compounds, or cholesteric esters. These features may be summarized as follows : 1. The molecule will be elongated and rectilinear. If the molecule has ‘[flat” segments--e.g., benzene rings -liquid crystallinity is often enhanced. 2. The molecule should be “rigid” along its long axis; double bond(s) are common along this axis of the molecule. 3. Simultaneous existence of strong dipoles and easily polarizable groups in the molecule seem important. The most pronounced liquid crystallinity effect is most likely to

occur if the strong dipole is on the molecular axis. 4. Weak dipolar groups a t the extremities of the molecule are of subordinate importance. A general pattern of molecular structures which show liquid crystallinity may be illustrated from the compilation in Table I. From this table one might conclude that molecules which form liquid crystals often consist of two or more benzene “nuclei” joined by groupings of two (or a multiple of two) atoms. There are generally one or more multiple bonds along the molecular axis. Illustrative types of end groups, y, which can be attached to the molecular skeleton are summarized in Table 11. Seither of these tables is meant to contain an exhaustive listing of all the types of molecules which have been reported. The reader is referred to Kast’s list of liquid crystalline compounds published in Landolt-Bijrnstein ( 1 ) to see the diversity of

compounds which form thermotropic liquid crystals. \Ye should not ignore aliphatic compounds even though the number of such compounds forming liquid crystals is small compared to the aromatic type. The monocarboxylic acids and their salts are the most common aliphatic compounds which are liquid crystalline. The simplest aliphatic compound that shows liquid crystallinity is 2,4nonadinoic acid. Recent advances in the synthesis of liquid crystals and the role of molecular geometry in liquid rrystallinity hare been described effectively by Gray ( 2 ) . The “state of the art” in the synthesis of liquid crystalline compounds has reached the point where chemists can make compounds to meet the specifications they are seeking. For example, compounds with large atoms for X-ray studies, compounds that respond readily to electric and/or magnetic fields,


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Report for Analytical Chemists

Table I. Composition of Central Group in Aromatic-Type Molecules Showing Mesomorphic State

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p-n-Butylbenzoic acid (double molecule)

1,4-naphthalenediamine

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p,p’-Diethoxybenzalcyclopentanone

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p.(p-Cyanobenza1)anisidine p-(p-Methoxybenzoxy)benzoic acid

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Benzal-p-aminoazobenzene

Anisal-p-aminoazobenzene

l-Benzalamino-4’phenylazonaphthalene

1-(4’-Anisalamin0)-4-phenylazonaphthalene

-CH=N-

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Table II. Composition of End Group in Aromatic-Type Molecules Showing

Mesomorphic State

y = End group CHs( CH2)n(C H3)2C H(CH2)nCHs(C HAOCH30(CHz)nOCI, Br, I CHs(CHz)nOCO-

CNNOzN Ht-

Type Normal alkyl Branched alkyl alkoxy Oxygen-broken alkoxy Ha logen Alkyl carboxylic acid esters Aryl carboxylic acid esters Cyano Nitro Amine, substituted amine

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Dibenzalnaphthalenediarnine

Dibenzalcyclopentanone

‘C=CH-

4,4’-Di(p-methoxybenzylideneamin0)dibenzyl p,p’-Diacetoxystilbene

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Type Diphenylet ha ne


compounds which are good solvents for nmr studies or good stationary phases in chromatography represent the kind of synthetic studies carried out in recent years. Optical Properties of Liquid Crystals

Microscopic Observations. A convenient way to identify a liquid crystalline material is to test it for birefringence. Liquid crystals are birefringent, unique for one-component liquids. Microscopic observation of the smectic liquid between crossed polarizers shows that this liquid behaves optically like unaxial crystals, such as calcite. It ex-

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hibits positive birefringence-i.e., the ordinary ray has the lower refractive index. This feature is a consequence of the molecular array, not of the properties of the molecules or of the structure of small regions. The nematic liquid is optically positive. The optical axis coincides with the preferred direction of the long axis of the molecule. It is not optically active (refer to bulk sample) but if it is placed between glass surfaces and one surface is rotated slightly, the deformation of the structure by adhesion to the glass surface may result in an optically active system. Optical Rotation of Cholesteric Structure. The unique architecture of the cholesteric liquid gives rise to a number of optical properties not exhibited by nematic and smectic liquids. For example, linearly polarized light transmitted perpendicular to the molecular layers will have the direction if its electrical vector rotated progressively to the left along a helical path. The plane of polarization of the light will be rotated to the left through an angle proportional to the thickness of the transmitting material. Such materials are optically active. Cholesteric liquids have the capacity t o rotate polarized light to a very great degree. A 1-mm section of the optically active substance, (-) 2methylbutyl- 4 - (cyanobenzylideneamino)-cinnamate a t 75 "C rotates polarized light of a wavelength of 658 mp 5300" and of a wavelength of 452 my 27, 650". Dichroism. The cholesteric structure exhibits dichroism, that is, one of the polarized components of light is selectively reflected in the structure much more strongly than the other. This property accounts for the characteristic iridescent color in the cholesteric liquid. The colors exhibited are dependent upon the temperature, the material, and the angle of the incident radiation. Structure of Thermotropic Liquid Crystals

For many yeam three structures of thermotropic liquid crystals have been accepted, namely nematic, smectic, and cholesteric. Rmecent findings have shown that the cho-

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lesteric structure is a special case of the nematic structure. However, because of certain unique optical properties of cholesteric liquids we shall identify the cholesteric liquid alongside the nematic. The molecular arrangements in two dimensions of the different classes of thermotropic liquid crystals are represented schematically in Figure 1. It should be understood that these schematics present a reasonable picture of molecular packing but not necessarily a true one. A real understanding of molecular arrangements will come only after extensive X-ray studies. I n Figure 1, molecules are represented by lines which imply that the molecules can be considered as having rotational symmetry; a line represents the orientation of the molecular symmetry axis ; the thermal fluctuation of the molecule is neglected in the drawings. Molecules aligned perpendicular to the plane are represented by dots. Nematic Liquids. Nematic liquids differ structurally from isotropic liquids in the spontaneous orientation of the molecules along their long axes, I n the absence of external forces, the preferred direction of the long axes of the molecules is not constant but generally varies continuously with position. The common texture encountered in the nematic liquid is the schlieren texture, commonly called the threaded texture. The threads result from a particularly strong dependence of the preferred direction on position in the vicinity of the axis of the thread. There may be no preferred direction in the axis of the thread itself. As stated previously the nematic liquid exhibits positive birefringence. The optical axis coincides with the preferred direction of the long axis of the molecule. The molecules in a nematic liquid are not all exactly parallel because of thermal motion. The extent of parallelism is measured by the ordering parameter, by the expression S = 1/2(3c0s2 e - 1)

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where 0 is the angle between the long axis of a molecule and the axis of preferred orientation, which coincides with the symmetry axis in uni-


formly oriented nematic liquids. The angular brackets suggest that the average value of cos2 8 is used. For perfect parallel orientation of molecules S = 1 and for the isotropic liquid S = 0. The value of S is strongly dependent on temperature and increases with decrease in temperature in the nematic liquid region. I n the nematic liquid the range of 8 values lies between 0.3 and 0.8. Nematic liquids are easily oriented by a magnetic field with the optical axis becoming parallel to the field axis. Nematic liquids can be oriented by surfaces. Rubbing glass surfaces in a given direction leads to molecular orientations in which the optical axis lies in the plane of the film and parallel to the direction of rubbing. Cleaning glass with certain chemicals-e.g., dichromate-sulfuric acid-leaves a surface on which the molecules arrange themselves so that the optical axis is perpendicular to the plane of the film. Cholesteric Liquids. The cholesteric liquid, like the nematic, has the parallel orientation of the molecules along their long axes but superimposed on the parallel orientation is a spontaneous and continuous twist; it is a twisted nematic structure. There is no entropy difference between the twisted and untwisted structures. An optically active molecule is necessary to form a stable cholesteric liquid. If a planar cholesteric structure with a uniform twist is prepared in thin layers, it will possess a high optical activity and will reflect circularly polarized light selectively. The ordering parameter, S, in the cholesteric liquids, is assumed to be the same as in nematic liquids. I n a homogeneously oriented layer of a cholesteric liquid, the preferred direction of molecular orientation is not constant over the entire volume but instead there are parallel planes in which the preferred direction is constant. Moving along a line perpendicular to the planes in which there is a constant preferred direction of the long axes of the molecules, the preferred direction rotates uniformly. The pitch of this rotation through 2~ is generally found to lie between 0.2 and 20p. The pitch is usually temperature dependent and the tern-

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perature dependence mlay be positive or negative. The twist in cholesteric liquids, which result from their unique molecular packing, can be observed in the optical characteristics of homogeneously ordered layers. Some of these optical observations are as follows: (1) Over a small wavelength range, about hR = Pn, where P is the pitch of rotation and n is the average refractive index; irradiation of the sample parallel t o the axis of rotation shows that one circularly polarized beam is transmitted while the other is totally reflected. No reversal of the direction of roatation is found. (2) Outside this small wavelength range, irradiation parallel to the axis of rotation gives a strong optical rotatory power. If the wavelength, hR, lies in the visible region, the total reflection gives rise to bright colors. Since the pitch of the helical structure is strongly dependent on temperature, the color observed may also represent a temperature identification. One can readily see the possibility of using cholestric liquids as a device to show directly the temperature distribution on a surface. The pitch of the helix may also be affected by the presence of a solute dissolved in a cholesteric liquid. Thus the reflection color could likely be used to detect the presence of substances dissolved in a cholesteric liquid. The use of cholesteric liquids to detect and identify chemicals has not been particularly promising although if only one component is being observed, there are cases in which solutes have been detected in amounts of the order of parts per million, When only one solute a t a time is used with a cholesteric liquid as the detector, certain solutes will give linear plots of reflectance as a function of concentration over short ranges of concentration. Selectivity is not good and two different solutes may give the same color patterns over comparable concentration ranges. If the subject in question were studied carefully by chemists, useful procedures might be developed for qualitative analysis and likely quantitative analysis of selected compounds. To develop a practical analytical procedure there are many

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parameters which need to be studied composition of solute and solvent and heats of solution. Smectic Liquids. Smeotic liquids have arrangements of molecules in layers which give them a stratified structure. The smectic textures always occur, as expected from their higher order, a t lower temperatures than the nematic and cholesteric liquids. Based on present knowledge, the ordering parameter, X, in smectic liquids is greater than 0.8, and the temperature dependence is relatively small. Homogeneously oriented smectic layers, like nematic liquids, show positive birefringence. I n thin, nonhomogeneously oriented films, the different smectic textures can be recognized microscopically. At least three different textures have been described and identified as A , B, and C. A recent publication identifies five different smectic phases ( 3 ) . Smectic A is the most common. I n the smectic A texture the centers of gravity of the molecules are arranged in planes perpendicular to the preferred direction of the long axes of the molecules. Thus, the long axes of the molecules are perpendicular to the optical axis. Within the layers, the molecules are randomly positioned so that one may think of having a two-dimensional isotropic liquid within layers. The details of molecular packing in -e.g.,

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smectic textures B and C remain to be defined. Before leaving a consideration of smectic liquids, we might mention that other patterns of molecular arrangements can be suggested. Of these we mention two, namely the tilted and twisted structures. The tilted smectic (Figure 1D) has been known for many years. It does not have ordering of a higher order than the smectic A liquid. The tilted smectic has one plane of symmetry and a two-fold symmetry axis perpendicular to i t ; the structure is optically biaxial. Enantiomorphs may result in those structures with a spontaneous twist. A schematic design of a twisted smectic is given in Figure 1E. The thermal relationships among the different phases found in thermotropic liquid crystals are outlined in Figure 2. There is no effort to outline transitions within a phasefor example, there may be a number of transitions from one smectic texture to another. The transitions identified in the figure are reversible. There are some cases in which a system undergoes a transition that results in a monotropic phase. This phase results in the supercooling process and occurs, for example, below the expected transition temperature from liquid crystal to solid state.

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Figure 2. Typical transitions encoun-

tered in thermotropic liquid crystalline systems I- 0 LAB-GLO.Mechanical cleaning I I .--_ I Name I C-N point. Crystai-nematic transition ternS-l point. Smectic-isotropic transition temerature I .----_----___________________________________I_ I E-S point. Crystal-smectic transition tem~ % ~ t u ~ ~ iSmectic-cholesteric nt. transition I Address I erature temperature E. Ch point. Crystal-cholesteric transition S-N point. Smectic-nematic transition temI .-_-_--_________________________________------. I I City State k-Ierature point. Nematic-isotropic transition temCh-l point. Cholesteric-isotropic transition ! temperature temperature

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ANALYTICAL CHEMISTRY, VOL.

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peratu re

Report Theoretical Aspects of Liquid Crystalline Structures

Two theories to explain the structure of nematic liquids have been proposed, namely the continuum theory and the swarm theory. I n brief, ithe swarm theory states that the molecules in a nematic liquid form aggregates in swarms containing on the average 1oj molecules. The molecules within the swarm lie parallel to one another. The directional components of the interaction between swarms is small, so the orientation of the axes of the swarms to one another is random, An external force-e.g., magnetic or electric field-can impose a uniform orientation. The swarms have a limited lifetime and molecules in a swarm can exchange with the isotropic environment or with those of other swarms. The strong light soattering properties of nematic liquid crystals can be easily explained by the swarm theory. The nematic structure has been found to be more continuous than expected when the swarm theory was proposed; thus, the concepts of the theory have t o be altered to accommodate the findings or a new theory has t o be proposed. The continuum theory is based on the assumption that a t every position in an undisturbed liquid crystal there is a preferred direction for the orientation of the longitudinal axes of the molecules. This preferred direction is supposed t o change continuously with position, except possibly a t special surfaces where the preferred direction is no longer defined. An example of a special surface where there is a discontinuity is the thread in the nematic structure. The continuum theory can be lapplied to cholesteric and smectic structures as well as to the nematic one. The assumption of the continuum theory leads one to expect thalt liquid crystals have elastic flexible structures and that the deformation energy can be calculated for a nematic structure from equations derived from the fundamental assumptions of the continuum concept. The theory has possible applications in predictions for viscometry and for propagation of orientation waves. Unfortunately, no

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quantitative data are available on wave speeds. Daha on shear flow are scarce and thus the results of the continuum theory are still in need of rigorous testing. Some Applications of Liquid Crystals in Chemistry

The cholesteric structure generated by a cholesteric ester is greatly dependent on the geometry of the molecule. What one says about the geometry of cholesteric esters is not necessarily applicable to noncholesteric molecules that form the cholesteric structure, A proposed packing pattern for a cholesteric liquid is shown in Figure 1B and a more detailed pattern is shown in Figure 3. The ring system in cholesterol is essentially planar but the ester chain, the alkyl chain, the methyl groups, and some of the hydrogen atoms on the rings are not in the plane of the rings. Notice that the long taxes of the molecules in any given plane are slightly oriented relative to those in an adjacent plane so that the units attached to the rings can be accommodated. This staggered packing gives a helical pattern which accounts for some of

the interesting optioal properties of cholesteric liquids. I n a cholesteric liquid the wavelength of maximum scattering of visible radiation depends on the angles of incidence and scattering if the temperature is fixed. I n many respects the cholesteric structure behaves like a multiple-layer interference filter but because of its internal structure, the higher orders are suppressed. The scattering of visible radiation from a cholesteric structure is analogous to scattering of X-rays from crystals. As pointed out previously, the wavelength of maximum scattering of visible radiation from a cholesteric structure will change with temperature or with materials that dissolve in it. The change in pitch of rotation in the helical structure brought about by change in temperature or by intrusion of a chemical will cause a change in the wavelength of maximum scattering. We have already mentioned potential uses of cholesteric liquid crystals taking advantage of these properties. The cholesteric structure might be used for detection of infrared radiation. The liquid crystal possesses the in-

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herent qualifications for such detection but the paactical aspects of instrumentation are still to be developed, Cholesteric liquid crystals have been used in microwave detection. A detector that displays color as a function of microwave intensity has been developed ( 4 ) . Microwave energies of about 200 mW/ cm2 have been detected. This is not the lower limit of energy detectability but shows the potential of liquid crystals for measuring low energy outputs. Many liquid crystalline materials are very good solvents. The inherent structural characteristics of liquid crystals make them good solvents for use in chromatography, infrared and ultraviolet spectroscopy, and nuclear magnetic resonance and electron spin resonance. Solute molecules dissolved in a liquid crystal will experience some orientation effects. It seems reasonable that the nematic liquid which oan be uniformly oriented very easily in a magnetic field, or in a thin layer on a surface, would be a good solvent for experiments where it is desirable to have oriented solute molecules. The anisotropic components of the spin interactions are easy to observe in nematic liquids and generally cause very marked changes in the nmr spectra in contrast with what is found in isotropic liquids. I n deuteron resonance, the important interactions are the quadrupole interactions between the electron cloud and the nucleus, and in proton and fluorine resonances, the important interactions are the direct magnetic dipole-dipole interactions between the nuclei. These interactions have no influence on the position and intensity of the resonance lines in isotropic solutions. I n brief, the nmr spectroscopy in liquid crystals differs from that in isotropic liquids in one fundamental aspect. I n isotropic liquids the direct interactions of nuclei are the major factors in establishing the structure of the spectrum. The anisotropy of the liquid crystals accounts for the difference. To explain the direct interactions characteristic of the liquid crystal, we quote from Snyder and Meiboom ( 5 ) (this article gives a good synopsis of nmr in crystals) :

“The physics of the direct interaction is simple: Each proton has Q, magnetic dipole and thus produces a magnetic field which interacts with the magnetic dipoles of the other protons in the molecule. A simple magnetostatic calculation of the interaction energy between two parallel dipoles shows that it is proportional to (3COS 28 - l)r3,where r is the distance between the nuclear pair and 0 is the angle between the direction of the dipoles and a vector connecting the nuclei. I n liquids, rapid molecular tumbling occurs, and the above angular factor must be averaged over all values of e, properly weighted. I n isotropic liquids the result of this averaging process is zero and the direct coupling is absent. I n liquid crystal solvents, the environment of a molecule is anisotropic, and thus different molecular orientations are no longer equally probable. The weighted average now required for the angular factor will no longer be zero. Note that only the intramolecular interactions (within a molecule) are non-zero; the intermolecular interactions (between nuclei in different molecules) still average to zero, as in isotropic liquids, and in contrast to solids where both intra- and intermolecular interactions are effective. The strong dependence of the direct interaction on internuclear distance and orientation provides the possibility of molecular structure determination. For a completely oriented pair of protons separated by 1 A, the direct dipolar coupling fields correspond to a shift of resonant frequency of order 100,000 Hz, as degrees of orientation of 10% and higher are common, the direct interaction is often much larger than the indirect interactions and the chemical shifts. I n order to get resolved N M R spectra, a uniform alignment over the whole NMR sample is required. The diamagnetic anisotropy of the liquid crystalline phase provides the force by which a magnetic field may uniformly align the solvent. The above considerations apply not only to the liquid crystal molecules, but also to molecules dissolved in a liquid crystal solvent. If the solute molecule is elongated (Le., has a lower symmetry than cubic), it too will be partially oriented. Typically direct proton interactions up to 5000 Hz are observed in solute molecules. These shifts are usually far greater than

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those due t o chemical shifts and indirect coupling. Thus the magnetic fields due to the direct magnetic dipole-dipole coupling of nuclei determine the major structure of these N M R spectra.” Nuclear magnetic resonance spectra have been used for structure determination of molecules, studies of anisotropy of chemical shifts, determination of quadrupole coupling constants of deuterons, and the study of intermolecular forces. A number of nematic liquids have been used as solvents in nmr studies; la few are shown in Table 111. Since S is temperature dependent, a uniform and constant temperature is important. This can most easily be accomplished near room temperature by use of mixtures. Examples of these mixtures are given in Table IV. The nematic liquids in Tables I11 and I V orient uniformly in a mlagnetic field with the symmetry axis parallel to the field which is essential for structure studies. Lyotropic liquid crystals formed from surfactants have also been used as solvents (6) in nmr studies. A mixture of 36% sodium decyl sulfate, 7% deuterated decanol, 7% sodium sulfate, and 50% water (all Table 111.

Nematic Liquids Used as Solvent6 for NMR Studies

Solvent Bis-4,4’-hexyloxyazoxybenzene I1 4-(4’-Ethoxyphenylazo)phenylhexanoate Ill Butyl 4-(4’-ethoxyphenyl)carboxyphenyl carbonate I V Butyl 4-(4’-methoxybenza la mi no)met hylci nnamate V 4-(4’-Ethoxyphenylazo)phenyl heptanoate VI 4-(4’-Ethoxyphenylazo)p henylpentanoate

I

Nematic ray e , C 80-128 70-126

57.5-81

58-76 68-117

79-125

H , : C , - O ~ C O O H

Table IV. uids

Mixtures of Nematic LiqUsed as Solvents for N,MR Studies Used for meas yements,

Solvent (wt. fractions)” 0.25 (VI) 0.75 (Ill) 0.4 (11) 0.6 (V) 0.4 (VI) 0.6 (11)

+ + +

C,

as low as 30 45 60

See Table Ill for identification of compounds designated here by Roman numerals


by weight) forms, a t room temperature, a birefringent system which has nematic-like properties. I n a magnetic field, the symmetry axis of the solvent orients normal to the field. Benzene in this lyotropic solvent orients parallel to the symmetry axis as i t does in thermotropic nematic liquids; the degree of orientation is of the same order of magnitude. Lyotropic solvents containing water have some advantages in nmr studies in that hydrophilic compounds will dissolve easily. Thermotropic smectic liquids of type A are not affected by magnetic fields but fairly uniform alignment of a smectic liquid can be obtained by cooling a nematic or isotropic liquid in a strong magnetic field. Yannoni (7) has used this procedure to study the structure of l,l”l”-trifluorotrichloroethane in the smectic liquid p - (2-n-propoxyethoxybenxylideneamino) acetophenone. In such a system it is possible to turn the sample and change the angle between the symmetry axis of the liquid and the magnetic field. Bond angles and relative bond distances in solute molecules can be determined effectively using liquid crystals as solvents. The most elaborate studies to date have been made by Snyder and Meiboom (810) and have determined the geometry of cyclopropane, cyclobutane, and bicyclobutane. Nmr techniques using liquid crystal solvents can be used to detect molecular association-e.g., hydTogen bonding. Since S in the nematic solvent increases with decreasing temperature, it is generally found that the orientation of the solute molecules also increases as the temperature of the system decreases. There are exceptions to this behavior. For example, methanol in shows S reaching a maximum value and then decreasing with a decrease in temperature (11). Such an abnormal behavior indicates some kind of molecular association, probably hydrogen bonding between SOlute molecules. This explanation of decreasing S values seems reasonable since one expects the number of associations to increase with decreasing temperature.

Report

;\Ienburement of polarization of electronic l~ands( 1 2 ) of solutes in neiiintic iiquid cryst'als has proved usc,ful ior the study of selected organic riiolecules arid transition Coinpounds metal conipleses. wliic.li have been stuclicd using this technique inc lucie ,hl-caroteiie! p azosyanisole, arid n-tetrabutyl;iii:lnioiiiuin his l,enzone-1,2-ditliiolato I col~:iltnte~II1).Gray e t a l . (1,; 1 liavct iisecl nematic liquid crystals ;is sol\.cmts to iiieasure the poition of absorpticln hands in the infr:ireci rt'gion. For example, they np:)ileci tlie techiiitjuc to 1111:~ !COi lo and I:iko n ~ t . The incIwisei1 reaction !':it(' m t y 1jc attribute11 to the stereo-clcc*ti\.c c1i:iractcr of the nematic sol\-c~nt. This : i 1 ~ nof study shows great promise for a new field of kii i c t i c : i n i l nircli:ini.~ti(~ studies. 'Tiit. usc of liquid crystals as static#n:ir?-p!~:iscs iii v a r o r phase chroni:itog.nphy elion-s promise for imp i ~ ~ - cacpnratioi~ d of certain isom ( w :i!id of siilwt;inc[+ from a mult icoinpoiicnt sy?tcm. Thermodynaiiiir 8l:it:i . - u r I i as 1ic:its of solution :in11 :zvtivity rocfficicmts of solutes i n lirliiiil cry-tnl 11sing the chroinatogr:iphicx tpchnique c:in he readily d et (wiii n Thc~st1p:iration of compounds on ~lii,c,iii:ito~i,nl,Iiic columns using (711 ,

liquid crystals as stationary phases was successfully accomplished by t.wo laboratories several years ago. Dewar arid Schroeder (15,16/ found that tlie m- and p-xylenes could be separated rather effectively using the smectic phase of 4,4'-di-nhexyloxazosybenzeiie as tlie dationary phase. Tlie retention time for the rn-isomer was found to be shorter thnn for the p-isomer. This observation has been explained by pointing out that the more linear shnpc of t h e p-isomer as conipared to the riz-isomer a l l o w it to fit more e:isily into tlie "lattice" of the liquid ;iI, thus making the retention of the p-i,sonier greater. Characteristics of liquid crystalline subst'ances which seem t o enhn lice select ivi t y in the separkit>ioii of compounds niay be sumniarized nftcr Den-:ir and Schroetier 115, 1 6 ) and Kelker ( 1 7 1 n s follows: 1. Long liquid crystalline range favors selectivity. 2. High t r mi s it ion t enip er a ture from liquid crystal to i liquid enhanres selec 3 . Smectic phase appears t o be most fnvoraljle for separation of components of n niulticomponelit system. 4. TTithin a liquid crystalline range, the l m t selectivity is found t o he in the lower temperature range of the phase. Typical esamplcs which illustrate these points are the success which Dc.n.:ir anti Schroeclcr (15, 16) had in separation the H I - :inrl p-isomers of mcthylani~oles,dibroiiiobenzcnes, diiiicthoxybenzenee, methyl toluates, and chloroncetoplienoiies on liquid crystalline phases a t lory tcmpcratures in the liquid crystalline range in contrast n.it1i limited wlcctivity a t the !iigher temperatures in t h e sanie liquid cry-dnls. Iiellrrr i 1 7 ) found that those liquid cryst 31s which h a d long mesophases with relatively high transition tcmperatiiiw at the liquid crystal t o i.*otropic liquid gave improved sel e c t i ~ i t yin the separation of mixturea. The m- and p-xylenes could he wparntcd more effectively oil p . p'-azosyphentole :it 140 "C than on hesyloxyazoxyhenzeiie at 75 " C . Iielker ( 1 7 ) was the first t o determine molar thermodynamic functions for liquid crystnls, using chromatographic techniques. H e found Circle No. 128 on Readers' Service Card


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discontinuity a t the phase transiion point on plotting the logarithm i f the specific retention volume for 1 given solute against 1/T (OK). (elker found that the enthalpies )f mixing solutes in liquid crystaline solvents are one to three kcal/ no1 larger than in the correspondng isotropic phases. Further, the :xcess molar entropy of solution was of the order of 7 cal/mol higher n the liquid crystalline phase than n the isotropic phase of the same solvent. This indicates that the ) d e r in the liquid crystalline phase s disturbed to a greater degree by ;he intrusion of solute molecules Lhan is the case with the isotropic liquid. Kelker observed that the activity coefficients for solution processes in mesophases are generally greater than one. Martire et. al (18) made some very careful studies of thermodynamic properties of solutes and of solvents which are liquid crystals. Using the normal aliphatic hydrocarbons as the solutes and cholesteryl myristate as the solvent, they found that the overall heats of solution of all solutes (except n-nonane) are positive, which is interesting, in contrast to the usual negative heats of solution of these solutes in smectic and nematic liquids. I n recent studies, Martire and Chow (19) found that surface effects a t the gasliquid crystal interface on chromatographic columms may be neglected if a sufficient thickness of the liquid crystalline film is used (greater than about 1000 A ) . Once we obtain good thermodynamic data on solutes in liquid crystalline phases, we should be able to develop good separations of position isomers-e.g., xylenes-cistrans isomers and maybe optical isomers. Effective separations of the components in multicomponent systems should also be accomplished. b

References in Article

(1) Kast,. W. in Landolt-BGrnstein, 6th ed., Spnnger, Berlin, 1960,Vol. 11, Part 2a, p 266. (2) Gray, G. W., Mol. Cryst. Liq. Crust., 7, 127 (1969). (3) Demus, O.,Kunicke, G., Neelson, J., and Sackmann, H., Z. Naturf., 23% 84 (1968). (4) Augustine, C. F., Electronics, June 24,1968,p 118. (5) Snyder, L.C., and Meiboom, S., Mol. Cryst. Liq. Cryst., 7,181 (1969).

(8) Meiboom, S., and Snyder, L. C., SCience, 162,1337 (1969). (9) Snyder, L. and Meiboom, s., J . Chem. Phvs.. (1967). ” , 47.1480 .

c.,

(10) Meiboom, S., and Snyder, L. C., J . Amer. Chem. Soc., 90,2183 (1968). (11) Saupe, A., Englert, G., and Povh, A:, “Advances in Chemistry Series,” Amencan Chemical Society, Washington, D. C., No. 63,1967, p 51. (12) Ceasar, G. P., and Gray, H. B., J . Amer. Chem. Soc., 91,191 (1969). (13) Gray, H. B., Ceasar, G. P., and Levenson, R. A., J . Amer. Chem. Soc., 91,772 (1969). (14) Bacon, W. E., and Brown, G. H., M o l . Crust. in oreas. “~~I&. Crust.. “ (15) Dewar, M. J. S., and Schroeder, J. P., J . Amer. Chem. S O C ,86,5235 (1964). (16) Dewar, M. J. S. and Schroeder,J. P., J . OTQ.Chem, 30,3485 (1965). ~~~~~

.

~~

.~.

(18) Maitire, D. E., Blasco, P. A.,

Carone, P. F., Chow, L. C., and Vicini, H., J . phys. Chem,, ’12,3489 (1968). (19) Martire, D. E., and Chow, L. C., July, 1969, private communic&tion. Selected General References (1) Brown, G. H., and Shaw, W. G., Chem. Rev., 57, 1049 (1957). (2) Brown, G. H., Dienes, G . J., and

Labes, M. M., “Liquid Crystals,” Gordon aud Breach, Science Publishers, Ino., New York, N. Y., 1967. (3) Gray, G. W., “Molecular Structure and the Properties of Liquid Crystals,” Academic Press, New York, N. Y., 1962. (4) Brown, G. H., Chemistry, 40, 10 (1967). . . (5) Saupe, A,, Angew. Chem. intern. Edit., 7.97 (1968).

Glenn H. Brown holds the Ph.D. from Iowa State University and has taught a t hhe universities of Mississippi, Iowa State, Vermont, and Cincinnati. At Kent State University since September. 1960, Dr. Brown is Director of the Liquid Crystal Institute and Regents Professor of Chemistry. H e is the author or coauthor of numerous papers in scientific journals, two patents and author or editor of six books. The scientific papers are quite diverse and cover the general subjects of polarography, conductance of solutions, viscosity of solutions, liquid crystals, liquid dielectrics, structure of liquids and concentrated s(dutions, and photochromism. His present research interests include liqLlid crystals, structure of concentrated salt solutions, and photochromism. In 1964 he was selected by his colleagues a t Kent St,ate University as the “Most Honored Faculty Member.” The Alumni As1wciation of his alma mater (Ohio University) presented him with a certificate of merit in 1966 and the Ohio Academy of Science honored him with a Distinguished Service Award the same year. Some of the organizations of which Dr. Brown is a member are the Ameri.= can Chemical Society, American Association for the Advancement of Science, Sigma Xi, Phi Lambda Upsilon, Alpha Chi Sigma, American Institute of Chemists, the Ohio Academy of Science, and the New York Academy of Sciences. He was president of the Ohio Academy of Science in 1960. Dr. Brown chaired the First International Conference on Liquid Crystals in 1965 and ithe Second International Conference on Liquid Crystals which was held August 12-16, 1968. Both conferences were held a t Kent State University. He has held important offi,ces and committee assignments in the Memphis, Cincinnati, and Akron Sections of the American Chemical Society. He was chairman of the Akron Section ACS in 1965 and now serves as Councilor from the Section. Dr. Brown has served on committees of the American Chemical Society a t the national level and was General Chairman for the Central Regional ACS meeting held in Akron, May 9-10, 1968. Dr. Brown is coeditor of the journal Molecular Crystals and Liquid Crystals. ~~

~~~

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Liquid crystals and some of their applications in chemistry - American

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